Centrifugal mill

ABSTRACT

A centrifugal mill having a feeder secured on a housing, the working surface of which has ribs and which accommodates a separator mounted on a shaft and arranged coaxially inside the housing to define an annular clearance with the working surface of the housing and having a cavity and a plurality of through holes in a side wall and balls placed in the cavity of the separator, some of the balls interacting during rotation of the separator with the ribs of the housing via the through holes in the side wall of the separator, the cavity of the separator accommodating a baffle defining with the separator an annular clearance of between 0.1 and 1.0 times the diameter of the ball, the annular clearance between the separator and working surface of the housing amounting to between 0.1 and 0.5 times the diameter of the ball, whereas the through holes in the separator have the form of slots.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to comminutaing materials, and more particularlyto centrifugal mills.

The invention can find application in the mining, construction andchemical industries for dispersing and mechanochemical activation ofsolids, and also in research facilities for modelling automatedproduction lines and for facilitating solutions associated with problemsbefore science and technology.

2. Description of the Related Art

There is known a centrifugal ball mill of continuous action (SU, A,395111) comprising a stationary housing having a cylindrical innersurface, a vibration feeding means, and separators rotatable on a shaftand having balls freely inserted into grooves and accurately locked inplanes substantially perpendicular to the axis of rotation by theseparators, the diameter of the balls reducing and the number of suchballs increasing in the direction of feeding the material to becomminuted.

Comminution of materials in a continuous action centrifugal ball mill isbased on crushing the material particles at the points of contact of theballs to which centrifugal forces caused by rotation of the separatorsare exerted with the cylindrical inner wall of the housing.

A characteristic feature of the centrifugal mill of this type is lowpower of impact pulses produced by the milling balls oscillatingexclusively due to the lack of stability of their travel on the layer ofmaterial moving along the inner surface of the housing; in addition, thematerial tends to stick to the cylindrical wall of the housing to form asoft base, which reduces communition efficiency, hampers the flow of thematerial, and affects the function of the balls.

There is also known a continuous action centrifugal mill (cf., SU, A,925386) comprising a feeder secured on a housing having a ribbed workingsurface and accommodating a separator arranged coaxially on a shaft andincluding a cavity filled with balls. A side wall of the separator hasthrough holes corresponding to the diameter of the milling balls whichenter the through holes during rotation of the separator to engage withthe ribs of the housing.

This centrifugal mill operates as follows. Centrifugal forces generateddue to rotation of the separator displace the balls to the inner wall ofthe separator and the balls are piled in a row of layers. Therewith,part of the balls in the layer adjacent to the wall enter the throughholes of the separator. These balls are caused to roll and slide on theworking surface of the housing periodically hitting the ribs, wherebythe impact energy is transferred to the rest of the balls present in thecavity of the rotating separator, which due to different geometrical anddynamic conditions of interaction move randomly. Importantly, thegreater the number of balls charged to the cavity of the separator, thehigher is the frequently of ball interaction and the smaller is thedistance they fly between impacts. Conversely, a reduction in the chargeof balls in the cavity of the separator results in less frequentinteraction between the balls, a greater distance they travel betweenthe impacts, and longer time periods between impacts.

The initial material delivered from the feeder to the cavity of theseparator is acted upon by the turbulent flows of air and randomlymoving balls, and is forced via the through holes of the separator,which are open at points in time following the impacts of the balls,against the ribs on the working surface of the housing. The material ispreliminarily treated in the cavity of the separator due to the dynamicinteraction of the randomly moving balls, whereas the main andsubsequent treatment stages of the material are based on collision ofthe balls against the ribs of the working surface of the housing.Therefore, the designation of balls randomly moving in the cavity of theseparator is limited exclusively to ensuring a more prompt return of theballs present in the through holes of the separator to the workingsurface of the housing subsequent to their recoil from the ribs. At thesame time, while performing this function, the balls present in thecavity of the separator obtain rotation pulses as they interact with theballs in the through holes to result in friction therebetween as well asin a loss of energy and wear of the balls. In addition, the ballspresent in the through holes of the separator are in contact with thewalls of the through holes about an arc, which also promotes losses ofenergy for friction. Reducing the number of balls in the cavity of theseparator causes less energy lost for friction, although extends thetime of flight of the balls present in the through holes to consequentlyresult in a reduction in the frequency of impacts delivered on theworking surface of the housing. The random movement of the balls presentin the cavity and in the through holes of the separator accelerates wearof the balls, slows down the comminution process and activationaccompanied by losses of energy and failure to provide stable periodicalvibration impact movement of balls at a high frequency of theirinteraction with the material being comminuted.

In addition, the heretofore described construction of the centrifugalmill in the horizontal or inclined positions fails to provide thesufficient level of activation and comminution fineness due tonon-uniform distribution of the material at the top and bottom parts ofthe working surface of the housing.

In the case of vertical arrangement of the axis of the mill, acceleratedvelocity of the material under the forces of gravity reduces thedwelling time of the material at the working surface to result in lessefficient material comminution process. When comminutins viscousmaterials, the output capacity of the mill tends to reduce, because thematerial adheres to the working surfaces to result in slowing down itsmovement toward the discharge hole. Also, this mill construction failsto provide uniform delivery of reagents directly to the working zone inthe desired percentage.

SUMMARY OF THE INVENTION

The invention aims at providing a centrifugal mill ensuring steady andperiodic vibration action of balls with a high impact frequencynecessary for efficiently carrying out processes associated withdispersion, mechanical and chemical activation of materials throughmodifying the mill structurally.

The aims of the invention can be materialized in a centrifugal millcomprising a feeder secured on a housing, the working surface of whichaccommodates a separator mounted on a shaft and arranged coaxiallyinside the housing to define an annular clearance with the workingsurface of the housing. The feeder has a cavity and a plurality ofthrough holes in a side wall, and balls are placed in the cavity of theseparator, some of the balls interacting during rotation of theseparator with the ribs of the housing via the through holes in the sidewall of the separator. The cavity of the separator accomodates a baffledefining with the separator an annular clearance of between 0.1 and 1.0times the diameter of the ball, the annular clearance between theseparator and working surface of the housing amounting to between 0.1and 0.5 the diameter of the ball, whereas the through holes in theseparator have the form of slots.

Preferably, the baffle of the centrifugal mill, according to theinvention, is mounted on a shaft arranged coaxially with the shaft ofthe separator.

The arrangement of the baffle in the separator, the side wall of whichhas slots, as well as the provision of annular clearances ensureperiodic vibration impacts of balls with a very high frequency ofimpacts delivered by the balls against the working surfaces toconsequently result in more efficient comminution and activation, aswell as in improved kinematics of the processes.

Preferably, the baffle has a cavity for feeding reagents thereto.

Advisably, the side wall of the baffle is provided with nozzles forfeeding the reagents from the cavity of the baffle to the cavity of theseparator.

Desirably, the nozzles are of variable cross-section.

Favorably axis of the nozzles are at an angle to the axis of rotation ofthe baffle.

Importantly, the nozzles in the side wall of the baffle are arrangedabout a helical line.

In view of the aforedescribed, the provision of the cavity in the baffleensures uniform delivery of reagents to the nozzles. The abovedescribedarrangement of the nozzles improves homogenization and cleaning of allthe working surfaces from the material sticking thereto.

Preferably, the baffle is fabricated from metal ceramics.

This reduces losses of energy from deformation as the ball hits thebaffle due to the increased modulus of elasticity of the metal ceramicsas compared with other materials, which in turn promotes an increase inthe share of energy expendable directly on the material treatment at theinvariable level of the overall amount of energy delivered for theprocess, thereby increasing the efficiency of dispersion and activation.

Advisably, each slot in the side wall of the separator diverges towardthe axis of rotation of the separator.

Favorably each successive slot is offset relative to the preceding slotand to an end face of the separator.

Such arrangement of the slots provides point contact of the balls withthe driving wall of the separator slot, prevents repeated collisions ofthe balls with the driving wall, and enables uniform distribution of theballs on the working surface of the housing to thereby reduce losses ofenergy from friction, slow down wear of all engaging surfaces, andextend the service life of the centrifugal mill.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail with reference toa specific preferred embodiment thereof taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is a longitudinal sectional view of a centrifugal mill accordingto the invention;

FIG. 2 shows a section taken along the line II--II of the centrifugalmill represented in FIG. 1;

FIG. 3 shows the position of a ball as it hits the rib of the housing, asection taken along the line III--III of the mill shown in FIG. 1; and

FIG. 4 shows the same as in FIG. 3 illustrating the position of the ballas it hits the baffle.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A centrifugal mill comprises a feeder 1 (FIG. 1) secured on a fixedhousing 2 having a pipe 3 for discharging the material, and a coolingcavity 4, an inner working surface 5 of which has ribs 6 (FIG. 2).Mounted coaxially in the housing 2 (FIG. 1) on a shaft 7 is a separator8 defining with the working surface 5 of the housing 2 an annularclearance 9. The separator 8 has a cavity 10 occupied by balls 11,whereas a side wall 12 has through holes in the form of slots 13.

The cavity 10 of the separator 8 accommodates coaxially a baffle 15 tothe shaft 7 of the separator, an annular clearance 16 being definedbetween the separator 8 and baffle 15. The annular clearance 9 amount is0.1 to 0.5 the diameter of the ball 11, whereas the annular clearance 16amount is between 0.1 and 1.0 times the diameter of the ball 11. Thebaffle 15 has a cavity 17, whereas the shaft 14 of the baffle 15 ishollow to facilitate feeding of reagents. The side wall 18 of the baffle15 has nozzles 19 of variable cross-section with axes of these nozzlesset at an angle to the axis 20 of rotation of the baffle 15, the nozzlesin the side wall 18 being arranged about a helical line 21. Each slot inthe side wall 12 of the separator 8 diverges toward the axis of rotationof the separator 8 coinciding with the axis 20 of rotation of thebaffle; each successive slot 13 is offset relative to the preceding slot13 and to an end face 22 of the separator 8, each such slot 13 beingfurther arranged in parallel with the axis of rotation of the separatorand having a wall 23 (FIG. 3), which is a driving wall.

The centrifugal mill operates in the following manner. A drive (notshown) rotates the separator 8 (FIG. 1). The balls 11 present in thecavity 10 are forced centrifugally to the periphery, that is the ballsare caused to enter the slots 13 made in the side wall 12 of theseparator 8 to spread uniformly along the slots 13 and initiate a steadyand periodic vibration impact movement in a space confined by theworking surface 5 of the housing 2, slots 13 of the rotating separator 8and side wall 18 of the baffle 15 rotatable through engagement with theballs 11.

The initial material delivered through the feeder 1 is caused by theaction of centrifugal forces and turbulent air flows to be thrown ontothe working surface 5 of the housing 2, spread uniformly thereon, andmoved to the discharge pipe 3, while being subjected to impactsdelivered by the balls 11. The material is therefore caused partially tobreak, and partially to store energy in the form of disturbed crystallattice, i.e., is subjected to activation.

In addition, gaseous or liquid reagents are delivered from a source (notshown) along the hollow shaft 14 to the cavity 17 of the baffle 15 to bethen fed onto the working surface 5 and to the annular clearances 9 and16 through the nozzles 19 of variable cross-section set at an angle tothe axis 20 of rotation and along the helical line 21. Havingaccelerated in the nozzles 19 of variable cross-section, the reagentsare thrown against the surface of the balls 11, wall 23, slots 13 of theseparator 8, and the working surface 5 of the housing 2 to act on thematerial adhering thereto and clean them, whereas the jets of gas orliquid tend to entrain the material providing thereby its movement,improving homogenization, and saving the reagents.

Kinematics of the balls 11 will now be considered. The separator 8 (FIG.3) rotates at an angular velocity ω₁, the wall 23 thereof forcing orpushing the ball 11, which after rolling on the working surface 5 of thehousing 2 collides with the rib 6 at a velocity u. Due to the impactengagement with the rib 6 the ball changes direction of travel, andmoving along the straight line 00₁ at a velocity υ hits the side wall 18of the baffle 15 to again change direction, moving along the straightline 0₁ 0₂ (FIG. 4) at a velocity υ₁ and colliding with the workingsurface 5 of the housing 2. Then the cycle of movement of the ball 11 isrepeated after hitting the next rib 6.

The provision of rigid baffle wall 15 (FIGS. 1, 2) mounted inside thecavity 10 of the separator 8 facilitates the return of the balls 11 tothe working surface 5 of the housing 2 subsequent to the impact on theribs 6, increases the frequency of engagement of the balls 11 with thematerial being treated, and improves the kinetic characteristics of theprocess of comminution or activation.

The baffle wall 15, fabricated from metal ceramics and containingtungsten alloys and posessing a high modulus of elasticity, reduceslosses of energy of the ball 11 associated with plastic deformationduring impact, which promotes an increase in the share of energyexpended directly for treating the material at an invariable amount ofenergy delivered to the machine, whereby the efficiency of dispersionand activation is increased.

Because the baffle wall 15 is secured on the shaft 14 coaxially with theshaft 7 of the separator 8, the impacts of the ball 11 on the side wall18 of the baffle wall 15 are accompanied by transfer of energy of themoving ball 11 to the baffle wall 15 to rotate the latter which acquiresan angular velocity ω_(o) at steady operating conditions.

Therewith, the transfer of energy from the ball 11 executingtranslational and rotational motion is minimized, since the linearvelocities of points at the surface of the ball 11 and points at thesurface of the side wall 18 of the baffle 15 are equalized, and relativeslipping at the contact points is reduced to consequently result in areduction in the loss of energy due to friction.

With the stable angular velocity ω_(o) of the baffle wall 15, the ball11 after collision with the wall 18 moves at an angle γ (FIG. 4) to aradius circumscribed through the point of impact to the working surface5 of the housing 2. As a result, the material being treated is subjectedto impacts from the balls 11 mainly at invariable relations betweencompression and shearing stresses.

In order to change the relationship between such stresses, the shaft 14imparts to the baffle wall 15 (FIG. 1) an angular velocity differentfrom the angular velocity ω_(o) and corresponding to conditions of freerotation of the baffle wall 15 heretofore described. With this aim inview, the shaft 14 is linked with a drive (not shown).

In the course of dynamic interaction with the baffle 15 the ball 11tends to change its angular velocity of rotation relative to its ownaxis and linear velocity in the direction of recoil to act on thematerial, thereby causing a change in relationship between thedirections of compression and shear, which results in changingconditions of comminution and activation, and consequently expands thefunctional capabilities of the proposed mill.

The annular clearance 16 between the baffle 15 and separator 8 amountingto between 0.1 and 1.0 times the diameter of the ball 11 is necessaryfor vibration impact movement. When the annular clearance 16 is lessthan 0.1 times the diameter of the ball 11, the separator 8 and thebaffle 15 get jammed by starting material particles with the result thatthe separator 8 starts to operate in jerks, which consequently causesunstable and non-periodic vibration impact movement of the balls 11whereby the efficiency of the process of comminution or activation isreduced.

Conversely, the annular clearance 16 of over 1.0 times the diameter ofthe ball 11 causes jamming of the balls 11 between the side wall 18 ofthe baffle 15 and the inner surface of the side wall 12 of the separator8, or, alternatively, may result in the movement of the ball 11 from oneslot 13 to another. In consequence, the vibration impact movement of theballs 11 may be disturbed to cause jamming of the balls 11 in the slots13.

Provision of the annular clearance 9 between the separator 8 and workingsurface 5 of the housing 2 of 0.1 to 0.5 times the diameter of the ball11 is dictated by the general relationship between the size of the balls11 and the particles of material being comminuted of approximately 1:10,that is with the size of the ball 11 of 100 mm across, the size of thematerial being comminuted is 10 mm. In addition, particle size of lessthan 0.1 times the diameter of the ball 11 causes reduced efficiency ofmaterial treatment, since in this case the reaction of the ball 11 onthe driving wall 23 will approximate the line connecting the axis ofrotation 20 and the geometrical center of the ball 11, and uponcollision with the particles of the material the ball 11 will tend totravel along the path of the least resistance, viz., it will be forcedto the slot 13, failing to comminute the material particles. If the sizeof the annular clearance 9 exceeds 0.5 the diameter of the ball 11, thelatter will operate under precompression, which will be accompanied byadded losses of energy due to friction between the driving wall 23 andball 11 and fast wear thereof.

In view of the foregoing, the provision of annular clearances within theaforedescribed range ensures dispersion and activation of the materialbeing comminuted accompanied by steady and periodic vibration impactsexerted by the balls at a high frequency.

In addition, the slots 13 of the separator 8 diverging toward the axis20 of rotation of the separator 8 and baffle wall 15 also ensurevibration impact movement of the balls 11, and reduction of losses dueto friction as the ball 11 hits the rib 6, to result in reduced wear ofthe wall 23 of the separator 8.

Displacement of the slots 13 relative to one another and to the end face22 of the separator provides uniform distribution of the balls 11lengthwise of the working surface 5 of the housing 2 during operation,which facilitates uniform wear of the working surface 5 and ribs 6 andextends the service life of the centrifugal mill.

The invention can be used in process lines associated with comminution,mechanical and chemical activation of solids, in automated orepreparation, and in preparation of comminuted samples forexpress-analyses with the aim of obtaining materials with newproperties.

What we claim is:
 1. A centrifugal mill comprising a feeder secured on ahousing having a working surface which has ribs and which accommodates aseparator mounted on a shaft and arranged coaxially inside the housingto define an annular clearance with the working surface of the housingand having a cavity and a plurality of through holes in a side wall, andballs placed in the cavity of the separator, some of the ballsinteracting during rotation of the separator with the ribs of thehousing via the through holes in the side wall of the separator,characterized in that the cavity of the separator accommodates a baffledefining with the separator an annular clearance of between 0.1 and 1.0times the diameter of each ball, the annular clearance between theseparator and working surface of the housing amounting to between 0.1and 0.5 times the diameter of each ball, whereas through holes in theseparator have the form of slots.
 2. A centrifugal mill as claim 1,characterized in that the baffle is mounted on a shaft arrangedcoaxially with the shaft of the separator.
 3. A centrifugal mill asclaimed in claims 1 or 2, characterized in that the baffle has a cavityfor feeding reagents thereto.
 4. A centrifugal mill as claimed in claim3, characterized in that a side wall of the baffle is provided withnozzles for feeding the reagents from the cavity of the baffle to thecavity of the separator.
 5. A centrifugal mill as claimed in claim 4,characterized in that the nozzles are of variable cross-section.
 6. Acentrifugal mill as claimed in claim 4, characterized in that axes ofthe nozzles are at an angle to an axis of rotation of the baffle.
 7. Acentrifugal mill as claimed in claim 4, characterized in that thenozzles in the side wall of the baffle are arranged about a helicalline.
 8. A centrifugal mill as claimed in claim 3, characterized in thatthe baffle is fabricated from metal ceramics.
 9. A centrifugal mill asclaimed in claim 3, characterized in that each successive slot is offsetrelative to a preceding slot and to an end face of the separator.
 10. Acentrifugal mill as claimed in claim 1, characterized in that each slotin the side wall of the separator diverges toward an axis of rotation ofthe separator.
 11. A centrifugal mill as claimed in claim 10,characterized in that each slot runs parallel with the axis of rotationof the separator.